Heat exchanging apparatus



Patented Jan. 12, 1932 UNITED STATES I PATENT OFFICE ARCHIBALD H. DAVIS, JR., PITTSBURGH, PENNSYLVANIA, ASSIGNOR TO SHAW- PERKINS MANUFACTURING COMPAN Y, POBATION OF PENNSYLVANIA 0F PITTSBURGH PENNSYLVANIA, A COR- HEAT EXCHANGING APPARATUS Application filed June 17,

This invention relates to the heatin of fluids by heat transfer from another uid, and especially to preheating of air by means of gases of combustion.

A major object of the invention is to provide a method of heating a fluid by absorption of heat through a heat exchanging wall from another fluid, which, as compared with the apparatus used,

' tion of cooled gases,

" tus for use in the practice of the method according to this invention, which is simple, rugged, easily operated may be constructed from standard materials and parts, and which is compact and highly eflicient in operation.

The preferred embodiment of the apparatus is shown in the accompanying drawing, which schematically shows a plan view, partly in section, of the apparatus.

The customary practice in preheating air or other fluid by means of gases of combustion passed through a heat exchanging apparatus has been to cool the gases in the apparatus to the stack exit temperature. The

incoming gases supplied to the apparatus are usually cooled somewhat by admixture with gases at a lower temperature, and for this purpose there is usually used a portion of the cooled gases leaving the apparatus, the balance of the cooled gases being passed directly to the stack. Although this procedure gives good heat transfer efficiency, the mean temperature difl'erence available for heat transfer is low, and consequently large heat exchanging areas are required.

I have found that if the hot gases supplied 1929. Serial No. 371,409.

supplementary heat exchanging surface for abstraction of a further portion of their contained heat, there may be obtained a much higher mean temperature difference between the gases and the fluid. Inother words, according to my invention, the fluid to be heated is passed through a heat exchanging apparatus having a main and a separate supplementary heating section. Hot gases are supplied to the main section where, instead of being cooled the stack exit temperature, they are but partially cooled. A major portion of these partially cooled gases is then recirculated with incoming highly heated gases, and the remainder of the cooled gases passes to the supplementary section and is there cooled to stack exit temperature. Because of the very much higher means temperature difi'erencethus secured, the heat exchanging area may be substantially reduced while attaining the same heating provided by prior practice.

Various types of apparatus may be used in the practice of the invention. For example, it may comprise separate unitary main and supplementary heatingsections adapted to have fluid to be heated flow continuously from one to another. It is preferred, however, to provide self-contained heating sections in a unitary apparatus, one form being shown in the accompanying drawing.

In the apparatus shown in the heat exchanging surface comprises a plurality of tubes 1 joined at their ends of tube sheets 2 and 3 mounted in a casing l in the usual manner. The disposition of the tubes and other features of this type of construction are well known and require no extended description.

This form of apparatus is especially adapted for preheating air, and preferably the air is passed through, and the heating gases over, the tubes. A partition member 5 associated with the outside of the tubes and connected to casing 4 divides the gas contact surface of the tube into separate main and supplemen- IOU tary heating sections 6 and 7. This partition and for most purposes it fits closely enough to the tubes to prevent material leakage, without being welded to them. As shown in the drawing, the main heating section comprises the major portion of the gas contact surface.

Hot gases enter the main section through an inlet provided on one side of a baffle'8, and after circulating over the tubes, leave this section through an outlet on the other side of the bafiie. Ba'flies 8a are also provided in supplementary heating section 7. All of these baflies may comprise tube sheets extending partly across the sections.

I-Iighly heated gases from a combustion unit are supplied to the inlet of the main section, and means are provided'for recircu-' lating with these gases a regulated portion of the partly cooled gases from the outlet of this section. To this end there is provided a return connection 9 between the outlet and inlet, provided with an intake 10 for fresh highly heated gases. A blower indicated schematically at 11 draws cooled gases from the outlet of the main section through the return, and passes them with fresh gases to the inlet, the amount of gases thus recirculated being readily controlled byregulation of the blower" operation.

The portion of the cooled gases not recirculated in the main section passes through a conduit 12 to supplementary heating section,

7, and circulates therethrough to. a flue connection 13. If desired, a'blower 14 may be inserted in the stack connection, to regulate the proportion of gases drawn to the stack.

Air to beoheated ispassed directly through the tubes, preferably counter-current to the direction of gas flow- That is, cool air is passed by a'blower 15 through a duct 16 into the end of the tubes heated by the supplementary heating section, where it is partly heated. The air is heated further, to the desired temperature, in-passing through the portion of the tubes disposed in the main heating'section, and is passed to any desired point by an outlet duct 17.

In using this apparatus in practising the method of the invention, the gases passed into the main heating section are cooled only to a point materially above that of the stack exit temperature. That is, only a portion of the heat contained in the gases is absorbed in the main section. A major portion of the partly cooled gases is drawn through return 9, and highly heated gases from a combustion'unit, for example an oil burner, enter the return from intake 10. The recirculated gases cool the fresh incoming gases, and the amount of recirculated gases is proportioned so that the temperature of the mixture is suitably high to give proper efliciency, but

passed to the stack through connection 13.

The advantages of the invention can be seenbest by consideration of a specific example,'such as 'a drying operation in which air at 600 F. is supplied to a drier where it is cooled to 300 F., the cooled air being then returned to the heat exchanger for reheating. By way of example, the temperature of gases of combustion may be taken to be 3400 F. In the prior practice, the gases passed through thepreheater are cooled to stack exist temperature about 380 F., and a portion of the cooled gases is continuously passed to the stack. The rest of the cooled gases recirculates, cooling the incoming gases to about 1000 F., at which temperature the mixture is passed into the heat exchanges. By means of formulae well known in the art it can be shown that the mean temperature difference available for heat transfer according to this for example to about 660 F. The gases not recirculated are passed to the supplementary heating section and there cooled to the same exit temperature as in the preceding example, namely 380 F. By calculation it is found that in this case the mean temperature difference is about 370 F. I

The substantially greater temperature difference available for heat transfer which results from the practice of the invention makes possible a material reduction of heat exchanging surface. Also, higher gas velocities may be used without increasing draft loss, thus permitting further reduction of heater surface. Thus a heater having a heating area of 14400 square feet operated in accordance with the invention under the conditions given in the foregoing example will heat the same volume of air per unit time as a heater operated under prior practice having 33000 square feet of heating surface. With the reduced surface afforded, the higher gas velocity does not increase the total draft loss of the heaters of this invention over that of prior heaters. The substantial reduction in heating surface also decreases construction costs and reduces the space required.

I claim: I

1. An air preheating apparatus comprising tubular heat exchanging elements associated in a unit, an inlet to one side of the tubes for air to be heated and an outlet for heated air a partition associated with the other side 0 the tubes forming a main and supplementary heating sections for transfer of heat to air passed over the opposite side, a hot gas inlet Q to the main section adapted to be connected to a source of highly heated gas, an outlet for cooled gases from the main section, means for recirculating a major part of said cooled gases with incoming highly heated gases, and a connection between said sections for passing the balance of the gases from the main section to the supplementary section.

2. An air preheating apparatus comprising tubular heat exchanging elements associated in a unit, an inlet to one side of the tubes for air to be heated and an outlet for heated air, a partition associated with the other side of the tubes forming a main heating section adjacent the heated air outlet, and a supplementary section adjacent the air inlet, a hot gas inlet to said main section adapted to be connected to a source of highly heated gases,

an outlet for cooled gases from the main section, a return connection between said gas outlet and inlet provided with means for recirculating a major part of the gases from the main section, a connection between said gas outlet and the supplementary section, and baflies in each of said sections.

3. A process of heating a fluid by transfer of heat from hot gases, comprising circulating hot gases continuously over one side of a heat exchanging surface, passing the fluid continuously over the other side of said surface and abstracting a portion of the heat from said gases, returning a major portion of the gases partially cooled by circulation over said surface and mixing them with incoming highly heated gases, and withdrawing continuously a volume of the partially cooled gases equal-to that of the incoming highly heated gases and passing them over one side of an -additional heat exchanging surface over the other side of which said fluid is con- .tinuously passed, said fluid flowing countercurrent to said gases.

In testimony whereof, I sign my name.

ARCHIBALD H. DAVIS, JR. 

